The present invention relates to a method of pretreating an epoxy resin substrate preparatory to electroless copper plating. More particularly, the invention relates to a method which, in performing electroless plating to form a printed circuit on a glass fiber cloth-reinforced epoxy resin laminate, makes it possible to form an electroless copper plating layer which is superior in peeling strength by applying a special pretreatment to said epoxy resin laminate.
As for methods of forming a printed circuit on a glass fiber cloth-reinforced epoxy resin laminate, there are two known methods, the copper cladding method which uses an epoxy resin laminate having a thin layer of copper bonded to the surface thereof in advance, and the so-called additive method which applies direct plating. In the case of the copper cladding method, substrates having sufficient properties to meet high-level requirements are available with comparative ease, but the method has unavoidable disadvantages, such as ununiform film thickness in through holes, the difficulty of forming high precision circuits, an economic loss to chemicals which will be incurred when the unnecessary portions of the copper cladding layer are removed by etching (it being noted that on an average, 70 to 80% of the copper cladding layer on the entire surface area must be dissolved for removal), and the necessity of disposing of waste liquid. In the case of the additive method, as far as applicability to epoxy resins is concerned, there has not yet been developed any chemical surface treatment technique which is as effective as those applied to phenolic resins, ABS resins, polystyrene, and polypropylene. One of the improved technique ever proposed is to roughen the laminate surface by shotblast. In lines 39-45, No. 8, Vol. 23 and lines 40-48, No. 9, Vol. 23 of "METAL SURFACE TECHNIQUES" published by the Metal Surface Technique Society of Japan in 1972, it is stated that the peeling strength can be improved by a treating epoxy resin surface with chromic acid after swelling it with a solvent. However, this method has not been put into practical use, for its industrial merit is low because of required equipment and disposal of waste liquid.
The principal object of the invention is to provide a novel method of pretreating an epoxy resin substrate preparatory to electroless copper plating to form a printed circuit by using an epoxy resin substrate, especially a glass fiber cloth-reinforced epoxy resin substrate, said method being capable of greately improving the peeling strength of electroless copper plating layers.